SETTLEMENT IN FLOW - UPSTREAM EXPLORATION OF SUBSTRATA BY WEAKLY SWIMMING LARVAE

Exploration may play a crucial role in habitat choice by larvae of ben thic organisms. Experiments testing exploratory behavior of larvae in flow were conducted with cyphonautes larvae of the bryozoan Membranipo ra membranacea. The response of the larvae to two flow parameters-flow direction and shear (i.e., velocity)-was examined by using two differ ent substratum morphologies (flat plate and cylinder), which induce di stinct flow patterns. Settling cyphonautes larvae possess a mechanism of locomotion, in addition to swimming, which may facilitate explorati on in flow conditions that otherwise would sweep the larvae away. The results show that cyphonautes larvae are able to explore substrata in all directions in flow velocities that are much faster than their loco motion speeds, and that their preferential motion is upstream. This be havior has been largely ignored to date despite its potential ecologic al importance. For instance, upstream exploration may enable larvae to locate specific, obligatory settlement sites by tracking waterborne c hemicals to their sources.